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ABSTRACT: Diagenesis of Brent reservoirs in the Hild Field, Norwegian North Sea: temperature, timing and origin of fluids

Girard, Jean-Pierre1, Harald Johansen2, Andy Canham3, Jean-Claude Lacharpagne4, and Frédéric Sommer5
(1) BRGM, Orléans, France
(2) IFE, Kjeller, Norway
(3) IRES, Ellesmere Port, United Kingdom
(4) ELF, Pau, France
(5) TOTAL-FINA, St. Rémy-les-Chevreuses, France

The deeply buried Brent reservoir sandstones of the Hild Field, Norwegian North Sea, have undergone intense diagenetic alteration. Fluid inclusion microthermometry, stable isotope analysis and K-Ar dating of major diagenetic cements combined with detailed petrography and burial history permitted to constrain the conditions and timing of diagenetic transformations as well as the nature and origin of diagenetic fluids.

Abundant precipitation of non-ferroan calcite occurred during early diagenesis at shallow depth. This early calcite cement formed at 20-30°C from predominantly marine, possibly brackish, water, and was subsequently remobilized (recrystallized ?) during burial. The subsequent development of vermicular kaolinite and pore-lining, platy illite, at intermediate burial depth occurred at temperatures between 40 and 95°C during Cretaceous time. The nature of fluids responsible for the formation of these cements could not be constrained.

Late diagenetic events, which were most damaging for reservoir quality, include quartz overgrowths, ferroan calcite, dickite and pore-bridging, fibrous illite. They occurred at 2.5 to 4.3 km depth and at temperatures of 95-155°C from Early Tertiary to present. These deep burial cements formed from evolved basinal fluids, isotopically similar to present-day formation fluids. The introduction of deep basinal waters followed an abrupt increase in subsidence rate at about 70 Ma, and preceded and accompanied the migration of hydrocarbons in the reservoir.

The diagenetic model developed for the Hild Brent is in general agreement with diagenetic models previously developed in several other North Sea fields. One main difference, however, is the absence of evidence for any major contribution of meteoric water.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia